Fluid power units typically drive pumps and injectors in hazardous locations such as oil fields and refineries. The usual practice in the oil field is to drive the power unit with natural gas. It is important not to vent gas from the power unit; for example, hydrogen sulphide is commonly a part of natural gas and is toxic to humans. Also, environmental concerns with emission of greenhouse gasses such as methane compel the reduction of gas venting.
Further, control valves for power units should be directly connected to the power cylinder to avoid breaks in connecting lines; indeed, connecting lines found in prior-art power units should be avoided all together.
There a need for a fluid power-unit having a closed system, where the power unit and its associated control valve do not vent to the outside, either at controlling poppet valves, or exhaust ports, or through leakage through seals made in construction of the units.